Benzophenone, photochemical reductive dimerization

The photochemical reduction of benzophenone to benzopinacol in the presence of ferrocene31 apparently relates to a high concentration of ketone so that sufficient unquenched benzophenone triplets are present and proceed to the dimer in the usual fashion. 

The pinacol reaction is an example of radical dimerization (Scheme 4.29). Stabihzed free radicals have sufficiently long hfetimes to permit coupling outside solvent cage confinement Scheme 4.30 shows two such coupling reactions. The first is the photochemical reduction of benzophenone to benzopinacol (Scheme 4.30a). The second is an example of the oxidative coupling of phenols, a transformation that is an important step in the biosynthesis of alkaloids (Scheme 4.30b). 

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